The present invention relates generally to a zoom lens system, and more particularly to a zoom lens system used on electronic cameras, video cameras, etc.
Lens systems used on electronic or video cameras, for instance, are required to have not only a reduced size but also a long-enough back focus so as to locate low-pass or color filters just in front of their image planes.
For zoom lenses of simple construction used on electronic or video cameras, etc., it has so far been general to use a so-called two-group zoom lens system comprising, in order from the object side, a first lens group having negative power and a second lens group having positive power so as to make sure of a back focus. To reduce fluctuations of chromatic aberrations due to zooming, however, it is required to introduce achromatization in each group. Thus, each group should be constructed of many lenses. For example, JP-A 6-11650 discloses a two-group zoom lens system that has an F-number of 3.3 to 4.7 and a zoom ratio of 1.8, and comprises seven lenses.
A zoom lens system has its focal length varying with a spacing change between lens groups. At the same time, there are changes of the height and angle of incident light rays on lenses, which result in fluctuations of aberrations. To reduce fluctuations of aberrations, especially longitudinal chromatic aberration and chromatic aberration of magnification, many lenses are used in each lens group.
Higher image qualities may be achieved by use of many lenses. However, this renders it difficult to achieve size, weight, and cost reductions.
A typical zoom lens system using aspheric surfaces to reduce the number of lenses involved is disclosed in JP-A 6-11650 mentioned above. The aspheric surface is a sort of means effective for making correction for spherical aberration, coma, and distortion, but can make no direct correction for chromatic aberrations. In addition, this zoom lens system still uses as many as seven lenses. Even when the number of lenses involved is reduced by use of aspheric surfaces, it is indeed difficult to achieve size, weight, and cost reductions because it is still impossible to eliminate chromatic aberrations.
Another zoom lens system designed to reduce the number of lenses by using a gradient index type lens, for instance, is disclosed in JP-A 2-56515. However, the gradient index type lens is generally difficult to fabricate.